Optical amplifiers are essential in optical communication networks for boosting optical signals weakened by propagation losses, splitting and filtering. There are two main types of optical amplifiers, fiber optical amplifiers and planar waveguide optical amplifiers. Fiber amplifiers are optical glass fibers having cores doped with optically active elements like rare earth elements such as erbium. Waveguide amplifiers are planar waveguides fabricated on suitable substrates such as silicon. The planar waveguide amplifiers include cores doped with optically active elements.
Fiber amplifiers are usually favored over waveguide amplifiers in most applications because waveguide amplifiers are typically much shorter, requiring a correspondingly higher level of doping to achieve the necessary gain of fiber amplifiers. Higher levels of doping adversely affect the efficiency of the amplifier. However, waveguide amplifiers are easier to manufacture than fiber amplifiers because large numbers of them can be fabricated on a single wafer and integrated on the same chip with other components making the whole unit less expensive. Therefore, in applications requiring relatively small components, device integration and ease of packaging, it is advantageous to use waveguide amplifiers.
It would be beneficial for waveguide amplifiers to have a high index of refraction contrast between the core and cladding of the waveguide to enable mode confinement, and a relatively small waveguide core cross-section for reduced power density to achieve inversion. Unfortunately, the light scattering at the core/cladding interface increases considerably with index contrast and therefore, degrades the performance of the amplifier.
Accordingly, a need exists for an improved waveguide optical amplifier that significantly overcomes the problem of light scattering.